Zeilinger had invited the Dalai Lama to his laboratory following a meeting at Dharamsala in Northern India last October at which he and four other physicists had, over the course of five days, discussed physics and cosmology with the Buddhist leader. In Dharamsala, Zeilinger had demonstrated some basic quantum phenomena - such as wave-particle duality - using a laser-based double-slit experiment with a photomultiplier tube connected to a loud-speaker. The Dalai Lama's visit to Innsbruck allowed other quantum effects to be demonstrated for him.

Zeilinger says that the Dalai Lama did not have a problem with photons having both particle and wave-like properties, but was reluctant to accept that individual quantum events are random. For example, he refused to accept that we cannot know which path a photon takes in a two-path quantum interference experiment. Zeilinger notes that continuity of existence is very important to Buddhists because it leads to reincarnation.

However, observation plays a key part in what we can know in both quantum theory and Buddhism, and Zeilinger was surprised to learn that the Dalai Lama agreed that there are not only limits on what we can measure, but also limits on what we can know, even in principle.

So what is Tibetan Buddhism? And what possible connection can it have with physics? According to Alan Wallace, an interpreter at the meeting, Buddhism is a spiritual tradition with strong empirical, philosophical and religious components, including a belief in the after-life and reincarnation and healthy doses of meditation. Buddhism, he explains, is based on the four noble truths: the reality of suffering and conflict; the inner origins of suffering and conflict; the possibility of the cessation of suffering and its sources; and that Buddhism presents a path to this cessation through spiritual practices. The bottom line is that the root of suffering and conflict is ignorance and delusion, and that the path to spiritual freedom is the path of knowledge and insight.

Wallace is well placed to discuss the links between science and Buddhism. After spending 14 years as a Buddhist monk in India, Switzerland and the US, he graduated in physics from Amherst College in the 1980s. It was at Amherst that he met Arthur Zajonc, the physicist who was the scientific co-ordinator for the Dharamsala meeting. Wallace is now professor of Tibetan studies at the University of California at Santa Barbara.

"It is natural for Buddhists to be interested in science, " he says, "because science is the most complete and successful theory of the physical universe we have. The Buddhist pursuit of truth includes not only the nature of consciousness, about which modern science knows very little, but also the entire world of which we are conscious."

Wallace agrees that quantum mechanics and Buddhism have many similarities: neither is fully objective (i.e. certain quantum properties only have meaning in the context of a measurement) nor fully subjective. Consciousness and various mind-body problems are also similar in this respect, he adds. According to Wallace, the Dalai Lama had not realized before that these sorts of philosophical questions could be so clearly demonstrated in the laboratory, while physicists were surprised that the introspective approach of Buddhism led to similar questions.

Zajonc says that he found many elements of Buddhism potentially quite helpful to the philosophical treatment of quantum mechanics. "It quickly became clear, " he adds, "that Tibetan Buddhism offers a vast and subtle set of philosophical approaches that we in the West would benefit by knowing, even in the sciences."

The conference in Dharamsala was the sixth in the "Mind and Life" series in which the Dalai Lama meets with scientists, but the first on the physical sciences. The previous five had been concerned with the brain, consciousness and related topics. According to Wallace, the meetings are a response to the Dalai Lama's own fascination with science and his belief that Tibetan Buddhism must not turn its back on modern knowledge. There are two main reasons for conferences: to provide a high-level tutorial for the Dalai Lama; and to encourage scientists to explore new ideas, inspired by Tibetan Buddhist philosophy, psychology and meditation.

George Greenstein, a theoretical astrophysicist at Amherst College, spoke about cosmology in Dharamsala. The other physicists present were Piet Hut of the Institute of Advanced Studies in Princeton, David Finkelstein of the Georgia Institute of Technology, Zajonc and Zeilinger.

"The dialogues, " says Greenstein, "included a good deal of factual information in which the Dalai Lama was extremely interested." Philosophical questions were also discussed. For example, is the big bang a moment of creation, as opposed to a transformation from one state to another? How can we understand creation? And did time and the laws of physics exist before the creation?

Greenstein says that philosophical questions are never answered, just discussed in new and interesting ways. His hope was that the Dalai Lama would ask the questions in new ways. "This happened in general, " he says, "but I cannot quote chapter and verse about specific topics. It was more amorphous and subterranean."

So why did Zeilinger agree to visit the Dalai Lama in the first place? "Science is an endeavour on which we have just started, " he says. "We are just fledglings and it is important to pull together all the intellectual traditions in the world."

Would Greenstein like to continue the discussions? "I would love to go back to Dharamsala, " he says. "Considering we were discussing all of physics and astronomy, we only scratched the surface. We flew over a new continent. I'd love to go back and land and walk around a bit."